A known device for coding video pictures is described in “Hardware Implementation of the Framestore and Data Rate Control for a Digital HDTV-VCR” presented at the HDTV Symposium in Japan, November 1992. The known device comprises a picture transformer for obtaining a series of coefficients which is representative of a picture block, a quantizer for quantizing the series of coefficients with a step size, and an encoder for coding the series of coefficients. Moreover, the known device comprises control means for controlling the step size in conformity with a target value for the number of bits per series of coefficients.
The device supplies a bit stream with a variable bitrate. To obtain a fixed bitrate per picture (or group of pictures), the known device comprises a buffer in which the bit stream is written at a variable bitrate and is read at the fixed bitrate. The quantization step size is controlled with the aid of the control means in such a way that the buffer maintains a desired fullness. The control means of the known device update the quantization step size per macroblock, i.e. one or more contiguous picture blocks.
A picture transform mode which is often used is Discrete Cosine Transform (DCT). This transform is performed with relatively small, contiguous picture blocks of, for example 8*8 pixels. The Lapped Orthogonal Transform (LOT) is currently in the limelight due to the absence of block artefacts. In this transform mode, the picture blocks partly overlap each other, for example by 50% in both the horizontal and the vertical direction. Notably for storage of X-ray angiographic pictures for medical applications, the LOT has appeared to be interesting. However, the X-ray picture characteristics are different from conventional video pictures. The dimensions of the picture blocks are therefore considerably larger. X-ray pictures having a dimension of 512*512 pixels appear to be optimally coded at a block size of 64*64 pixels with an overlap of 50%. After transform, this yields 256 blocks of 1024 coefficients each for each picture.
A problem of the known device is that with its step size control a fixed bitrate cannot be satisfactorily obtained per picture if the number of blocks is too small. The average bitrate is constant over a large number of pictures, but the bitrate may fluctuate to an unwanted large extent per picture. For comparison: a normal video picture comprises several thousand picture blocks. With such a number, a fixed bitrate per picture can be obtained within a margin of, for example 0.5%.